Ceramic composition for aircraft spark plugs



Patented Sept. 9, 1947 CERAMIC COMPOSITION FOR AIRCRAFT SPARK PLUGS Eugene Wainer, Niagara Falls, N. Y., assignor to The Titanium Alloy Manufacturing Company,

New York, N. Y., a corporationof Maine No Drawing. Application March 18, 1942, Serial No. 435,130

Claims. (01. 106-57) This invention relates to electrically insulating refractory compositions resistant to corrosion by gasoline fuels containing lead oxide. More particularly, it relates to such compositions which are suitable for use as ceramic insulators for spark plugs in aircraft engines and similar high compression internal combustion engines where gasoline containing lead compounds are used.

Ceramic insulators for automotive spark plugs have been extensively used for many years. Generally, the type of ceramic compositions used in automobile internal combustion engines also serve fairly well for aircraft engine use, as long as they are used in low power, low compression, and low speed engines. However, in recent years the horsepower output per unit weight of aircraft engines has been greatly increased, with the result that the motors operate at considerably higher temperatures, higher speeds and higher compressions. Under these conditions, the usual type plugs become less and less useful. Coincidental with the development of more highly powered engines, the elimination of piston knock by means of gasolines treated with tetra-ethyl lead has introduced further problems of such a nature as to render an automotive spark plug useless in the modern aircraft engine. For example, when the full power of a 1000 horsepower aircraft engine is suddenly developed, as in quick take offs or rapid climbs, temperatures up to 1800 F. are developed in the vicinity of the ceramic insulator used in the plug. At the same time the lead compound in the anti-knock gasolines decomposes to lead oxide, which deposits on the red hot ceramic insulator, with the result that extremely corrosive conditions are obtained. Under such conditions, the usual automotive plug is destroyed for all practical purposes in a very few minutes.

For the above reasons, ceramic compositions having highly unusual properties are required for satisfactory service as spark plug insulators in the modern aircraft engine. Of these unusual properties, one of the most important is the necessity for complete resistance to corrosion by molten lead oxide. This property is normally evaluated by immersing the suitably prepared composition in molten litharge at 1800 F. for a period of 24 hours. Compositions showing 5% or less volume decrease under such a test are considered suitable. Besides this property, the body mustbe an electrical insulator at elevated temperatures. It must be capable of vitrification at commercially obtainable temperatures (i. e., its maturation temperature must not be too high) and must have high thermal conductivity, high mechanical strength, and high resistance to thermal shock. Finally, the composition must be refractory in nature, that is it must not melt or decompose at the high temperatures encountered in aircraft engines.

In accordance with this invention, compositions have been developed which admirably fulfill these requirements. Such compositions com- .prise fired mixtures of zirconium oxide and magnesium oxide, in proportions ranging from one mol of zirconia to two mols of magnesia, to two mols of zirconia to one mol of magnesia. In such compositions the zirconia and magnesia together,

and usually the zirconia alone, comprise at least per cent of the dry composition, even when other ingredients are present. For compositions containing less magnesium oxide than a 1:1 molar ratio, the silica content should be as low as possible, while at ratios of 1:1 or higher, silica up to 10% of the total may be included without materially affecting the lead resistance.

Since combinations of zirconia and magnesia have proved so successful, combinations of zirconia with other group II oxides, such as oxides of beryllium, calcium, strontium, barium and zinc were also tried, without magnesia, but unsuccessfully. For example, among other disadvantages, combinations of zirconia with calcium oxide, strontium oxide or barium oxide require such exacting control of the firing temperature as to be commercially impracticable. Beryllium oxide is too costly for commercial use, and also requires ahigher temperature to fire or mature the bodies (maturation temperature) without any compensating advantage. Combinations of zinc oxide with zirconia alone have too short a firing range. However, under certain conditions, the addition of small amounts of other group II oxides, particularly barium and zinc, to the magnesia-zirconia combinations has been found to be beneficial in reducing maturation temperatures. In addition, the presence of zinc oxide appears to markedly reduce the wettability of the ceramic composition by molten lead oxide. In any case, the molal percentage of any of these other group II oxides should always be less than either the magnesia or zirconia.

Besides the essential ingredients zirconium oxide and magnesium oxide, there may also be present a certain amount of silica. Its presence in the compositions has several decided effects, some beneficial and some detrimental, so that the permissible amount of silica is quite critical. Thus, it increases the mechanical strength of the fired body, while/relatively small quantities markedly reduce reaction and maturation temperatures. By its use, a considerably tighter degree of vitrification is obtainable at a lower temperature than otherwise. At the same time the use of silica in the composition seriously affects the general lead oxide resistance ofthe finished body. In generaLas low a silica content as pos-. sible should be present when the molar ratio of magnesia to zirconia is less than 1:1, and the silica should not be more than by weight oi the composition in any case. It is preferred that the higher silica contents be used only with a 2 to 1 ratio of magnesia to zirconia.

Under certain conditions, the presence. of alumina in the composition is desirable. Like silica, it aids in increasing the mechanical, strength of the body. In addition, it does, not seem to be harmful as far as resistance to corrosion by lead oxide is concerned. It does, however, considerably increase maturation temperatures. The. result is that not more than 2.5%.

aluminashould be present, if silica is present;.. and

not more than 10%, if silica is; absent.

Although combinations of zirconia and mag.- nesia with small amounts of other oxides (A1203, SiOzand group II oxides) may be used, it. is preferred to use combinations of pure zirconia and magnesia for use in spark plugs. Since electrical resistance is important, alkali metal com-v pounds and heavy metal compounds, such as iron and manganese, should be kept to a minimum. In any case, the total amount of these impurities. in the composition should be less than 1 per cent.

The compositions of the present invention are prepared by calcination of mixtures of the finely divided oxides, or ofcompounds which form the;

oxides at elevated temperatures, such as carbonates and the like. Soluble salts other than the halides may be used asstartingmaterials except in the case of barium and strontium. Where these elements are present, the sulfate radical should notbe present due to the sluggishness with which sulfur is eliminated on calcination. Any" pure form of zirconium oxide or other zirconium compound forming zirconium oxide at elevated temperatures maybe usedas a starting material. Particularly suitable (except of course where barium or strontium is present) are the water insoluble oxysulfates of zirconium described in copending applications Serial Nos. 404,875 and 404,876, because of their high purity-and the state of subdivision and reactivitythese materials afford. To react suitably, the initial zirconium oxide preferably should have an average particle size of 1 micron or less, or the initial= material should be a compound of suchnature that on calcination a zirconia of particle size 1 micron or less is obtained. Zirconium oxysulfate is particularly suitable because on calcination to the oxide a particle size of 0.1 to 0.2 microns can beobtained.

The suitably compoundedmixtures are prepared by calcination to 2400- to 2600* F. until reaction is complete. Normally, an easily disintegrable calcine is obtained. The calcine is. comminuted to pass a 200- mesh screen and is then formed bya variety of procedures into the necessaryshapes. The compositions may-be pressed, rammed, jiggered, extruded orslip oast. Pressingor' ramming can be done with theaddition of a water solution of dextrine, gelatiineor similar organic bonding agent toyield the necessary green strength. Casting or jiggeringforming is done by means; of: additions of" minute quantities of thisbeing particularly useful when the body is to be extruded. In adding oxalic acid as a tempering' agent, the. deflocculants and water are added. first, andthe oxalic acid is then added to the thoroughly mixed batch. An amount of defiocculating agent equivalent to 0.05 to 0.5% of the batch is sufiicient for casting, and the acid addition for extrusion or jiggering is 0.1 to 1.0%.

After forming and shaping, the body is dried and fired in a clean oxidizing atmosphere to maturation. For the bodies described above, in no case; does the maturation temperature exceed 3100 15. Finally, the material ispolished to form the ceramic for spark plug insulators.

The invention having been described, the following examples will point out specific methods of practicing the same:

Example 1 192 grams of. basic magnesium carbonate containing 42.0% MgQ is thoroughly mixed with 200 grams of dry zirconium oxysulfate containing 61.5%. Z1102. The mixture is calcined at 2600? F. forthree hours. It is-then disintegrated by assage through a high speed swing hammer mill.

The calcine is then prepared for slip casting as follows: 100. grams is mixedv with 22 cc. of Water to which is added 0.07 gram. of NaePzOr. After casting in plaster of Paris molds, the piece is removed and dried and fired to 3050 F. in three hours, maintained at peak temperature for 1 v to 2 hours, and thencooled.

The same calcine can be used for jiggering' or extrusion by mixing grams of the powder with 15 to 19 cc. of water to which 0.05 gram of NQAPZO? has been added. After thorough mixing, 0.1 gram of finely powdered oxalic acid is completely kneaded in and the doughlike mass then worked into shape. The mass is dried and fired as above.

Example; 2

192 grams of basic magnesium carbonate and 200grams of zirconium oxysulfate as in Example 1' is thoroughly mixed with the addition of 6to 16 grams of *325 mesh silica, preferably of the flour type. The same. procedure is followed as in Example- 1, except that the presence of the silica enables the calcination temperature of the composition to be lowered 50 to degrees, and the maturation temperature 50- to 150 degrees, depending on the quantity of silica. The body may be prepared for slip casting as follows: 100- grams of calcineis mixed with 20.0 co. of water, 0.1 gram i of NH4OH and 0.1 gram of tannic acid. Thismixtureis suitable for plaster mold casting. Themix may be extended or jiggered by reducing.- the initial water' to 15 cc., mixing thoroughly, and adding 0.2 gram of powdered oxalic acid and kneading thoroughly.

Example 3.

80.6 grams of ..325 mesh MgO. (prepared from calcination of the precipitated carbonate). plus 1213 grams ofZrOz (prepared by low temperature calcination of zirconium carbide or cyanonitride) are mixed with 20 grams of silica flour and 15 grams of light grade aluminum hydrate containing 65% A1203. The mixture is calcined at 2400 F. for three hours. On cooling, the disintegrated calcine is formed by pressing from a mixture containing 100 grams of calcine and 12 cc. of gelatine solution. The body is fired to 2850 to 2900 F. for maturation and vitrification.

Example 4 96 grams of basic magnesium carbonate and 200 grams of zirconium oxysulfate as in Example 1 are mixed with 6.0 grams of Georgia kaolin plus 5 grams of silica fiour. This mixture is calcined at 2400 F., formed and shaped as in any of the preceding examples, and fired at 2900 to 2950 F. for maturation.

Example 5 41 grams of -325 mesh pure MgO and 123 grams of ZrOz (formed by calcination of zirconium oxysulfate at 1600 F.) are mixed with 13 grams of precipitated BaCOs and 3.5 grams of silica flour. In place of BaCOa, 6.5 grams of precipitated CaCOs or 9.7 grams of SrCOa may be used. The mixture is calcined at 2400 F., formed and shaped as in any of the preceding examples, and fired at 2850 F. for maturation.

Example 6 41 grams of 325 mesh pure MgO is mixed with 123 grams of ZIO2 (formed by calcination of pure zirconium carbide or cyanonitride). To this is added either 13 grams of precipitated BaCOz, or 6.5 grams of precipitated CaCOs or 9.7 grams of precipitated SrCOa. 4 grams of zinc oxide and 3.5 grams of silica flour are then added, the composition mixed thoroughly and calcined to 2400 F., for reaction. The calcine is shaped and formed as in any of the preceding examples, and then matured at about 2800-2850 F. This body has a short firing range between maturation and plastic flow.

Example 7 41 grams of -325 mesh MgO and 123 grams of pure ZrOz are mixed with 20 grams of BaCOz and calcined to 2450" F. for reaction. Maturation takes place at 2800 to 2850 F.

Example 8 41 grams of MgO plus 123 grams of mm plus 13 grams of BaCOa plus 7.5 grams of aluminum hydrate plus 5 grams of silica flour are mixed together and calcined to 2400 F. for reaction. The body matures at 2800 F.

In addition to use in aircraft and similar high compression internal combustion engines, the ceramic compositions of the present invention may be used in forming containers for the fusion of lead and its alloys, for which suitable containers have always been a problem when the temperatures exceed the melting point of lead by a few hundred degrees. Furthermore, since the electrical resistance is relatively high at elevated temperatures, and since the thermal conductivity at such temperatures is also high, the compositions are also useful for the embedding or packing of resistance wires in the assembly of electric cooking range elements, hot plates, soldering irons and the like.

When parts or percentages are mentioned. parts and percentages by weight are understood.

As many variations are possible within the scope of this invention, it is not intended to be limited except as defined by the appended claims.

I claim:

1. A spark plug having a ceramic insulator comprising a fired mixture containing a major proportion of zirconium oxide plus magnesium oxide, the molal proportion of zirconium oxide to magnesium oxide being between 2 to l and l to 2.

2. A spark plug having a ceramic insulator comprising a fired mixture containing a major proportion of zirconium oxide plus magnesium oxide, the molal proportion of zirconium oxide to magnesium oxide being between 2 to 1 and 1 to 2, said composition containing less than 10 per cent silica and less than 25 per cent alumina.

3. A spark plug having a ceramic insulator comprising a fired mixture containing a major proportion of zirconium oxide plus magnesium oxide, the molal proportion of zirconium oxide to magnesium oxide being between 2 to 1 and. 1 to 2, said composition containing less than 10 per cent barium oxide.

4. A spark plug having a ceramic insulator comprising a fired mixture containing a major proportion of zirconium oxide plus magnesium REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,682,251 Riddle Aug. 28, 1928 1,324,546 Ferngren Dec. 9, 1919 1,728,748 DAns Sept. 17, 1929 1,818,903 Martin Aug. 11, 1931 2,040,215 Rava May 12, 1936 2,172,839 Francis et a1 Sept. 12, 1939 2,098,812 Pulfrich Nov. 9, 1937 1,418,372 French June 6, 1922 

